B-cell Chronic lymphocytic leukemia (CLL) is the most common form of adult leukemia. Although a curative therapy has not yet been conceived, several distinct drugs have been shown to be effective in controlling the disease for periods of many months to many years in some but not all patients. At the present time the order of use of these drugs is determined by tradition, and their effectiveness is shown by trial and error. An in vitro test has been designed to prospectively determine the sensitivity of, CLL to the available drugs. In this test CLL cells are stimulated to divide by the combination of antigens and growth factors preferentially effective in B cell populations. Combinations of fluorescence-labelled monoclonal antibodies are used to discriminate CLL from other blood cells so that the relative effects of drugs against these normal and neoplastic cell populations can be accurately determined. Pre- and post treatment multichannel cytofluorometric analyses will be conducted in parallel with single drug trials using the same agents. The concordance or lack of concordance thereof will provide clear in pretreatment drug selection.